Scala Unable to pimp NaturalTransformation in scalaz or cats - scala

Question

For some reason the following fails to work
object NtExtTest {
implicit class NaturalTransformExt[M[_], N[_]](val self: NaturalTransformation[M,N]) extends AnyVal {
def test(b:Boolean) = b
}
}
when I call the method test on a natural transform. Intellij recognises it as an extension function, but the compile gives value test is not a member of cats.~> . The same happens when using the scalaz NaturalTransformation. Is there something I can do to help the compile recognise the extension?
Scala version is 2.11.8
An example which fails:
import NtExtTest._
class NtTest[B] extends NaturalTransformation[Either[B,?], Xor[B,?]] {
def apply[A](fa: Either[B, A]): Xor[B, A] = {
fa match {
case Left(l) => Xor.left(l)
case Right(r) => Xor.right(r)
}
}
}
val test = new NtTest[String]
test.test(false)
(above uses kind projector plugin, but equally fails with type lambdas, or single param higher kinded types)

Answer 1

Probably relates to SI-8286
object NtExtTest {
// this will work
implicit class NatTransExt1[E](val t: NaturalTransformation[Either[E, ?], \/[E, ?]]) {
def test1(b: Boolean): Boolean = false
}
// and this will work
implicit class NatTransExt2[E](val t: NtTest[E]) {
def test2(b: Boolean): Boolean = false
}
// but this will not work, because NaturalTransformation[F, G] and
// NaturalTransformation[Either[E, ?], \/[E, ?]] of different kind
implicit class NatTransExt3[F[_], G[_]](val s: NaturalTransformation[F, G]) {
def test3(b: Boolean): Boolean = false
}
}
I.e. it is not related to NaturalTransformation. It does not work in simple case as well:
trait SomeType[F[_]]
class SomeConcreteType[A] extends SomeType[Either[A, ?]]
object SomeTypeExt {
// this will not compile
implicit class SomeTypeEnrich[F[_]](val t: SomeType[F]) {
def test1: Boolean = false
}
// this will
implicit class SomeConcreteEnrich[A](val t: SomeType[Either[A, ?]]) {
def test2: Boolean = true
}
}
If the goal to extend NtTest, probably NatTransExt1 would be best choice if one would like to stay as generic as possible. NatTransExt2 is ok, if extension really specific to NtTest.